How to set blender default properties in addons?
A way of automate it could be to use an Application Handler, I think one of the scene update handlers would be the best fit. As you only want to alter the value once I would have the handler remove itself once it has done it's task.

As batFinger pointed out, it would also be useful to setup a persistent handler that will set the value when another file is loaded.

bl_info = {"name": "Set property", "version": (1, 0), "location": "blender",
    "description": "Test a way of setting a default property.",
    "warning": "Only testing", "category": "Test",
    }

import bpy
from bpy.app.handlers import persistent

@persistent
def setIt(scene):
    scene.world.use_sky_paper = True
    bpy.app.handlers.scene_update_pre.remove(setIt)

def register():
    bpy.app.handlers.scene_update_pre.append(setIt)
    bpy.app.handlers.load_post.append(setIt)

def unregister():
    bpy.app.handlers.load_post.remove(setIt)

if __name__ == "__main__":
    register()